Environment assisted cracking (EAC) has long been recognized as a major cause of component and structural failures, but the basic mechanisms of this process are still not fully understood. EAC includes hydrogen embrittlement (HE) and stress corrosion cracking (SCC) and other forms of metal and alloy cracking caused by the combined action of stress and environment. The development of standardized and practicable methods of testing and monitoring these degradation mechanisms is critical to understanding the kinetics of such failures and also to provide maintainers with an indication of operating conditions that have the potential to produce EAC in their systems.
One proposed mechanism of EAC is hydrogen embrittlement (HE), which is the process by which various metals, such as high-strength alloys, lose ductility and crack due to exposure to hydrogen. The process is associated with hydrogen uptake in the alloy, which can be initiated by atomic hydrogen produced by electrochemical processes. The hydrogen atoms affect the ductility of the alloy and promote the propagation of transgranular and intergranular cracks with reduced overall elongation to failure. The combination of atomic hydrogen, alloy properties, and applied stress can result in embrittlement and crack propagation. A wide range of metals and alloys are susceptible to HE, including high-strength low-alloy steels and nickel based materials.
Hydrogen embrittlement can occur during various manufacturing operations or operational uses, i.e. anywhere that the metal comes into contact with atomic or molecular hydrogen. Processes that can lead to HE include cathodic protection, phosphating, pickling, and electroplating. Other mechanisms of hydrogen introduction into metal are corrosion, chemical reactions of metal with acids, or with other chemicals, notably hydrogen sulfide in sulfide stress cracking, a process of importance for the oil and gas industries.
There are two ASTM standards for testing embrittlement due to hydrogen gas. The standard ASTM F1459-06 Standard Test Method for Determination of the Susceptibility of Metallic Materials to Hydrogen Gas Embrittlement (HE) Test1 uses a diaphragm loaded with differential pressure. The test ASTM G142-98 (2004) Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Environments at High Pressure, High Pressure, High Temperature, or Both uses a cylindrical tensile specimen tested in an enclosure pressurized with hydrogen or helium. 1 This and all other referenced publications below are expressly incorporated by reference herein.
Another ASTM standard exists for quantitatively testing for the Hydrogen Embrittlement threshold stress for the onset of Hydrogen-Induced Cracking due to platings and coatings from Internal Hydrogen Embrittlement (IHM) and Environmental Hydrogen Embrittlement (EHE) [1]—ASTM F1624-06 Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel by the Incremental Step Loading Technique.
The main interest of the environment assisted cracking detection system is for monitoring high-strength fasteners and structures that may be susceptible to HE under non-ideal cathodic protection conditions. Available hydrogen, in conjunction with high tensile loads and local stress risers that are characteristic of typical bolting applications (e.g., threads, surface imperfections), can result in EAC and subsequent fastener failure that has significant safety and availability implications for a variety of marine structures such as ships, oil and gas platforms, and pipelines. An in situ EAC sensing device would therefore provide valuable early warning capability, alerting users to environmental conditions that are prerequisite to cracking within the monitored structure.
Conventional in situ EAC sensing methods approach the problem in various ways. Of these, the methods involving the sensing of mechanical strain relief under cracking conditions are of particular interest. As one example, U.S. Pat. No. 3,034,340 to Jankowsky et al has proposed fracture specimens from sections of pipe, known as c-rings, to produce a large tensile stress in a notched region when loaded with an instrumented bolt. Crack initiation and propagation in the notched region tends to relax the initial bolt load, and thus is measureable by monitoring the load (strain) within the bolt member. Another prior proposal in U.S. Pat. No. 7,387,031 to Perrin et al includes a similar approach where strain gage instrumented flat strips of metal are deformed and held in a U-shape. Material loss caused by corrosion, erosion, pitting, and cracking are detrimental to the stiffness of the metal sample, thereby resulting in a deflection that is observed by the strain gage instrumentation.
In order to reduce the equipment burden, permit field deployment, and reduce costs, it is an objective of the technology described herein to produce a compact device that can supply the necessary stress to induce sample failure under approximate plane-strain conditions. The sample arrangement chosen for the current design is the circumferential notched tensile (CNT) geometry. As described in Ibrahim, R. N., et al. “Validity of a new fracture mechanics technique for the determination of the threshold stress intensity factor for stress corrosion cracking (KISCC)and crack growth rate of engineering materials”, Engineering Fracture Mechanics 75 (2008) 1623-1634, the CNT geometry is the smallest possible geometry that can produce approximate plane-strain crack loading conditions, within 3% of the ASTM compact tension (CT) specimens. To produce valid plane-strain conditions, the sample dimensions must be sufficient to constrain the plastic zone ahead of the crack tip. The CNT specimen can be made smaller thanks to its continuous circumferential notch, which affords a highly constrained plastic zone. The CT specimen, on the other hand, must be much thicker to ensure that the plane stress conditions at the free surfaces are small compared to the plane strain region in the interior of the specimen. For example, acceptable results have been obtained with 9.5 mm and 15 mm CNT specimens for materials that required CT specimen dimensions up to 80 mm.
Application of conventional monitoring and warning systems throughout the flooded or wetted spaces of a vessel or other structure would be complex, expensive, heavy, and vulnerable to damage. There is a current need for a simple monitoring system that can be used in the vicinity of critical high strength components to indicate the cumulative impact of conditions that can lead to EAC and premature failure. The problem of monitoring for conditions leading to EAC, in particular HE is solved according to the technology disclosed herein by utilizing a small CNT specimen in conjunction with very stiff sensor construction and highly sensitive strain gage instrumentation to provide high resolution crack depth measurement on the surrogate sample.
The technology described herein is embodied in novel sensors and methods for detecting the presence of conditions that would lead to environment assisted cracking (EAC) within structural components. According to certain embodiments, a sensor is provided which contains a material sample of similar composition to the monitored structure and is placed under a tensile preload that mimics the loading experienced by the monitored structure. Cracking within this surrogate sample correlates to damage in the monitored structure. The crack depth measurement is made by comparing the real-time tensile force on the sample to its initial value. Cracking in the sample increases its compliance and causes the load to drop in a predictable manner. The sensor design embodied by the technology described herein combines a very compact geometry with high-resolution crack depth measurement at a low cost, thereby making it very well suited for field installations, especially for alloys and conditions that have very low crack velocities that would normally go undetected.
Certain embodiments of the invention have the ability to monitor the initiation and progression of cracks in a tensile specimen with high resolution while in a small, ruggedized package, not requiring a large load frame or costly instrumentation. Additionally these devices are not susceptible to changes in fluid conductivity that can confound crack depth measurement techniques based on sample electrical conductivity. One principal design consideration that permits the high-resolution crack depth sensing is the high mechanical stiffness of both the sample and loading frame that enhances the load drop (sensed parameter) with crack growth. The sensor device according to embodiments of the invention is applicable both to real-time condition monitoring of a structure (e.g. underwater pipeline fasteners under cathodic protection) as well as laboratory characterization of the EAC susceptibility of materials, particularly during alloy development when a large number of tests are required for extended periods.
According to preferred embodiments, a sensor unit provides surrogate determination of crack development within a component of interest associated with a monitored structure. The sensor unit most preferably includes a sample sensor bolt having a shank with a threaded end. The shank of the sensor bolt is formed of a material serving as a surrogate of the material forming the component of interest associated with the monitored structure. A frame surrounds the shank of the sensor bolt and has fluid ports therein to allow fluid to contact an exposed portion of the sensor bolt shank in registry therewith. A load cell is operatively connected to the sensor bolt. A pre-load nut is threaded onto the threaded end of the sensor bolt shank and contacting an end of the frame so as to place the sensor bolt under an initial tensile stress. Crack formation within the sensor bolt shank caused by fluid acting upon the exposed portion thereof responsively relieves the initial tensile stress of the sensor bolt which is thereby sensed by the load cell. In such a manner, the crack development occurring in the sensor bolt shank may be determined and correlated to crack development occurring within the component of interest associated with the monitored structure.
These and other aspects of the present invention will become more clear after careful attention is given to the following detailed description of the preferred exemplary embodiments thereof.